The present invention relates to a magnetic carrier made of Li--Mn ferrite for use in a two-component developer which is used for developing electrostatic latent images formed on a surface of an image-bearing member (photosensitive drum) of image-forming apparatus such as laser beam printers, dry copying machines, etc. The present invention further relates to a two-component developer containing the Li--Mn ferrite carrier. The present invention still further relates to a method of developing electrostatic latent images by using a two-component developer containing the Li--Mn ferrite carrier.
In electrophotographic recording apparatus, it is known that a visual toner image is produced by the successive steps, for example, of (1) forming an electrostatic latent image corresponding to original image or information data, for instance, on a photosensitive surface of a cylindrical photosensitive drum (image-bearing member), (2) magnetically attracting a magnetic developer on a rotating developing roll equipped with an inner permanent magnet and disposed opposing the photosensitive drum, (3) delivering the magnetic developer, while forming a magnetic brush, to a developing zone where the electrostatic latent image on the photoconductive drum is slidingly brushed with the magnetic brush to produce toner images. The developed image is then transferred to a recording sheet and fixed thereon by heating or applying pressure
As the developer, one-component developer containing only a toner or two-component which is a mixture of a toner and a magnetic carrier has been used. In the two-component developer development, when the toner and the magnetic carrier are mixed together in a predetermined mixing ratio, the toner and the magnetic carrier acquire triboelectric charges of opposite polarities to allow the toner to cling to the magnetic carrier by electrostatic attraction. The magnetic carrier electrostatically retaining the toner is then supplied on the surface of a developing roller to form rotating magnetic brushes. The photoconductive surface containing the latent images is brought into brushing contact with the rotating magnetic brushes. During the brushing contact, only the toner is deposited on the image areas by electrostatic attraction between the latent image and the toner to produce visual toner images.
Generally, the two-component developer is required to have a good stability in image quality and to maintain such a stability against a continuous developing operation. To attain this long stability, it is necessary to regulate the ability of the carrier to providing the toner with triboelectric charges and the electric resistance of the developer within a suitable level. In addition, it is desirable that the ability and the electric resistance are not changed even in a long developing operation.
The toner is usually produced by blending a binder resin with several additives such as a colorant including dyestuff and pigment, a magnetic powder, a charge controlling agent, a wax, etc. to disperse the additives in the binder resin. To make the electric resistance of the developer stable, it has been considered to select suitable binder resin, charge controlling agent and other additives. As the material for the carrier, iron powder and a ferrite powder have been mainly used. To ensure a stable ability of providing the toner with a suitable amount of triboelectric charges, several considerations have been made in oxidation treatment of iron powder, material for the ferrite powder, the surface property of the powder, materiel for a coating, a method of coating, etc.
The iron carrier is usually produced by subjecting to its surface a suitable treatment such as oxidation treatment. The iron carrier subjected to such a treatment physically or chemically changes its surface conditions in a developing operation of a long period of time to make the toner cling thereon, thereby causing several defects of a decreasing ability of providing the toner with a sufficient triboelectric charges, a reduction in the resolution of produced images due to a high susceptibility to a moisture of the environment, and a short durability.
The ferrite carrier has become to be used in place of the iron carrier because it is chemically more stable than the iron carrier, it less changes its electric resistance during the use and its apparent density is about 2/3 of that of the iron carrier.
The ferrite carrier is a sintered product of a complete mixture of an iron (III) oxide and an oxide of at least one element such as Ni, Zn, Mn, Mg, Cu, Li, Ba, V, Cr, Ca, etc., and the practically used may include an Ni--Zn ferrite, an Mn--Zn ferrite and a Cu--Zn ferrite.
However, Zn, Ni, etc. are noxious to human body and the disposal of waste ferrite carrier containing such elements is legislatively regulated. The Li--Mn ferrite is not needed to be legislatively regulated, and therefore, such an advantage of the Li--Mn ferrite carrier is recently noted in the art. However, since the optimal chemical composition and the optimal developing conditions have not yet been established, it is difficult at present to stably reproduce images of a high quality by the use of a developer containing the Li--Mn carrier in a continuous developing operation.
In laser beam printers, digital copying machines, etc. which are coming to be widely used, a reversal developing method is usually employed, in which the latent image is formed by exposing a uniformly charged photoconductive surface to lights to discharge the image areas and remain nonimage areas charged. In the reversal developing method, a toner charged to the same polarity as that of the photoconductive surface is deposited on the latent image having a potential of about zero volt. Therefore, a high bias voltage is required to be applied between the developing roll and the photoconductive surface to sufficiently deposit the toner on the latent image. If the specific volume resistivity of the carrier is low, the carrier is likely to adhere to the photosensitive surface. On the other hand, a solid black images of uneven density is reproduced due to a strong edge effect if the specific volume resistivity of the carrier is high. Therefore, the carrier is required to have a low specific volume resistivity in the developing zone where the carrier is exposed to a high electric field due to the high bias voltage. Also, it is desirable for the carrier to have a high specific volume resistivity in the region other than the developing zone, because the charge amount of the toner is reduced due to leak if the specific volume resistivity is low. Thus, the specific volume resistivity of the carrier is preferred to be high in a low electric field, and low in a high electric field. However, the carrier in the prior art is insensitive to the change of the electric field.
Generally, the ability of the ferrite carrier to provide the toner with triboelectric charge largely depends on the chemical composition of the carrier. The known Ni--Zn ferrite, Mn--Zn ferrite, Cu--Zn ferrite, etc. provide a polyester-based toner of a strong negative chargeability with too much amount of charge which results in a reproduced image with insufficient density and a charge-up. Also, such ferrite carriers are less effective to charge a positively chargeable toner such as a styrene-acryl-based toner, and produce images with background foggings.